The immune system, when it is working properly, protects the individual from infection and from growth of cancers. In order to carry out these functions, it must be able to recognise and mount an attack against foreign antigens (including cancer-specific antigens), but not against self antigens present on normal cells throughout the body.
It is possible to stimulate the immune system in order to improve its level of protection. Vaccines, including single-protein antigens such as diptheria toxoid, are widely used to generate immunity against a specific antigen and thus a specific disease associated with that antigen. Where general stimulation of the immune system is desired, this can sometimes be achieved with nonspecific agents such as adjuvants, interleukins, interferons, and colony stimulating factors.
Occasionally, the immune system loses its critical ability to distinguish self from non-self. The resulting immunological assault on the individual's own tissues can take the form of autoimmune disease: for example, systemic lupus erythrematosis, Type 1 diabetes, or rheumatoid arthritis. In such a case, or alternatively where the individual is the recipient of a transplanted organ or tissue, suppression rather than stimulation of the immune response is desirable.
Non-specific down-regulation of the immune response is typically achieved by treatment with corticosteroids, azathioprine, cyclosporine, tacrolimus (FK506), rapamycin, or mycophenolate mofetil. Certain immunoglobulins, including the monoclonal antibody OKT3, have also been used for this purpose. Suppression of immunity against a specific antigen, called "tolerance induction", may also be possible. Methods that have been used for inducing tolerance against a particular antigen include intravenous or repeated topical administration of the antigen in dilute form, treatment with a very high dose of the antigen, and oral administration of the antigen.